1. Field of the Invention
This invention relates generally to the continuous, liquid phase oxidation of an alkyl aromatic with an oxygen-containing gas in the presence of an oxidation catalyst, and more particularly concerns a method for effecting this known oxidation process in stages in a single reactor.
2. Description of the Prior Art
Because of the great commercial importance of the oxidation of alkyl aromatics, it is highly desirable to improve the yield and quality of aromatic carboxylic acids produced thereby. It has been discovered that the use of lower process temperatures and higher oxygen partial pressures in this oxidation process affords selectivity and product quality benefits. Lower process temperatures and higher oxygen partial pressures favor the oxidation reaction over competing reactions which lead to the formation of undesirable products which reduce the yield and purity of the aromatic carboxylic acids produced.
The liquid phase oxidation of an alkyl aromatic is a highly exothermic reaction. Prior art processes for the oxidation of alkyl aromatics in the liquid phase to form carboxylic acids are generally performed in vertically disposed cylindrical reactors with a substantial portion of the heat generated by the exothermic oxidation being removed either by indirect heat exchange between the reaction mixture and a suitable cooling medium, either via coils located within the reactor or via circulating the reaction mixture through an external heat exchanger or by directly evaporating solvent in the reaction mixtures. The remainder of the heat generated results in an increase in the temperature of the reaction mixture. When the total oxidation is performed in a single reactor in which the reaction mixture is well-mixed, the temperature of the reaction mixture is determined principally by the total amount of heat generated in the oxidation less that amount of heat removed by indirect cooling or by solvent evaporation and, except for variances resulting from imperfect mixing of the reaction mixture within the reactor, the temperature of the reaction mixture is substantially the same throughout the reactor.
One technique to reduce the average reaction temperature would be to stage the oxidation in such a manner that the reaction temperature increases progressively through a series of stages. In this way the early chemical steps in the oxidation, which are the most susceptible to undesired side reactions, are conducted at low temperatures in order to maximize selectivity; and the final steps are conducted at high temperatures in order to increase rates and minimize the required reactor volume. In prior art staged oxidation of alkyl aromatics in vertical reactors, a separate reactor has been employed for each stage; the reaction mixture passes through the series-connected stages continuously; and in each stage, the oxygen-containing gas flows from the bottom to the top. By suitable regulation of the operating pressure and/or heat removal means in such staged reactors, the desired temperature increase from initial to final stages can be accomplished.
However the use of a separate reactor for each stage would require multiple reactor vessels, heat removal systems, and control and safety systems. Furthermore, for air oxidations in which heat removal is effected by means of solvent evaporation, the pressures will increase from stage to stage, and pumps are necessary to transfer the reaction mixture between stages. Such a system is also limited in the oxygen partial pressures available in the early reaction stages because of the lower operating pressure of those early stages and the necessity of limiting the oxygen content of the off-gas to avoid explosive mixtures.